Taken from page 14 of the FAQ.
If Chain Lightning is reversed by a Reverse Attack enchantment, the attack is redirected to the last target of Chain Lightning, not to the caster.
There is more information on Chain Lightening in the entry, but this is the part pertinent to the discussion.
Exactly this came up during last nights game. It's still a little unclear, though, what exactly happens: on the one hand no creature may be targeted twice by Chain Lightning. On the other hand, the FAQs state, that in the case of Reverse Attack the Attack is being redirected to the last target of Chain Lightnining (which already has been targeted). Now what happens:
1) does it actually take damage a second time?
And, more importently: how does the Attack go from there?
2) May Chain Lightning look for another viable target, which hasn't been targeted yet, or is it immediately cancelled upon revealing Reverse Attack and resolving its consequences? And in case it's not allowed to deal damage a second time, shouldn't then CL be cancelled right away?
3) Say, Reverse Attack is being revealed right on the first target. Then the Attack is being redirected at the caster, who now takes damage. But what then? Does it continue to zap to another target?
4) And when Block is revealed somewhere during Chain Lightning, does it cancel the whole Chain Lightning-Attack right away? I think it should, because the card clearly states, that CL only continues to zap, when it has dealt damage AND another viable target is around.
I'm a bit confused here.